The present invention relates to a method for controlling the intake airflow to an internal combustion engine and to a control circuit for implementing control methods therefore. More particularly, the invention relates to method(s) for controlling the intake air flow to an internal combustion engine of a supercompound-type. This type of engine has a driven output shaft, an intake air pipe, an exhaust gas pipe, a compressor located in the intake air pipe, and a turbine located in the exhaust gas pipe. There is a transmission system that connects the turbine, compressor and output shaft for energy transfer between the turbine and compressor, between the turbine and output shaft, and between the output shaft and compressor. The transmission system includes a variable transmission between at least two of the components including the compressor, turbine and output shaft, and a control mechanism or means for controlling the gear ratio in the variable transmission.
When using internal combustion engines, it is of interest to ensure that as much as possible of the energy produced by combustion can be utilized for the intended purpose. This has resulted in the manufacture of turbocharged engines in which energy from the exhaust gases is utilized for compressing the air on the inlet side of the engine, whereby the energy conversion by the engine can be increased. Turbocompound-engines are a further development in which the exhaust gas energy can be returned to the output shaft of the engine, whereby energy can be recovered from the exhaust gases. A further development of these two above-mentioned concepts is called a supercompound-engine in which energy from the exhaust gases can be returned to the output shaft, and can be utilized for operating a compressor located on the inlet side of the engine. By means of the connection between shaft, compressor and turbine, energy also can be transferred from the output shaft to the compressor in order to obtain a faster response of the turbocharging pressure increase on the inlet side of the engine than when only a turbocharger is utilized.
Supercompound-type internal combustion engines include an output shaft driven by the internal combustion engine, an intake air pipe, an exhaust gas pipe, a compressor located in the intake air pipe, and a turbine located in the exhaust gas pipe. A transmission system is connects the turbine, compressor, and output shaft for energy transfer between turbine and compressor, between the turbine and output shaft, and between the output shaft and compressor. The transmission system includes a variable transmission between the output shaft and compressor, and a control means for controlling the gear ratio in the variable transmission.
Furthermore, in connection with compressor-charged engines, the charging pressure is regulated by means of comparing measured pressure on the inlet side of the internal combustion engine with desired charging pressure calculated from given vehicle data, such as, and for example, desired acceleration, engine speed, and other types of engine and performance characteristics. In these cases, the control algorithms include such concepts as P (proportional), PI (proportional and integrating), PID (proportional, integrating and derivative), and LQ (Linear Quadratic).
It has been found that the control algorithms that are utilized in supercharged systems are unreliable when they are utilized for controlling the flow of inlet air to internal combustion engines of the supercompound-type. This results in the control system becoming slow and unstable, causing undesired pressure fluctuations to occur on the inlet side of the internal combustion engine. As a result of these undesired pressure fluctuations, problems with controlling the air/fuel-mixture to achieve the correct proportions will also arise, something that is of importance, for amongst other reasons, to reduce nitrogen oxide emissions from internal combustion engines of the diesel-type.
A first object of the present invention is to provide a method for controlling the flow of intake air to an internal combustion engine of the supercompound-type that enables a fast and stable control process in which the occurrence of undesired pressure fluctuations is reduced.
This object can be exemplarily achieved by means of creating a control signal for controlling a variable transmission between at least two of the components including the output shaft, turbine, and compressor at least partly from a signal based on the difference between a desired and an actual value of a condition variable in the intake air pipe, and partly from a signal based on the rotational speed of the compressor and/or turbine. In this way, a control system is obtained where changes in the considerable rotational energy stored in the turbine and/or compressor is taken into consideration, which implies that the control method reduces the risk that undesired fluctuations of the intake air flow are generated.
Another object of the invention is to provide a control circuit for implementing the described control method or strategy.